In image registration, an optimal transformation is sought between different image acquisitions of one or more objects. Image registration techniques may be used in the medical field to relate a pre-operative image of a patient's anatomy to a near real time image of the patient during actual treatment. During radiosurgery, for example, the change in target position at the time of treatment, as compared to its position at the time of the diagnostic treatment planning, may be detected. This may be accomplished by registering the 2D image acquired at treatment time with the 3D scan obtained at the time of treatment planning. A robust and accurate 2D-3D image registration algorithm may enable the position of the target, as viewed in the real-time 2D image, to be properly correlated with the pre-operative 3D scan. In practice, a formal mathematical transformation may be derived that best aligns the pre-operative image coordinate system with the patient's physical world coordinate system, defined for example in the treatment room.
Fiducial markers may be attached to, or implanted within, the patient before the pre-operative images are acquired, in order to accomplish a point-based alignment of the different coordinate systems. These fiducial markers are typically designed so that they can be localized relatively accurately in the pre-operative image, as well as in the real physical world. The respective localization points may then be used to calculate a rigid body transformation between the two coordinate systems.
The tracking of fiducials can be difficult for the patient, however, for a number of reasons. For example, high accuracy tends to be achieved by using bone-implanted fiducial markers, but less invasive techniques such as skin-attached markers or anatomical positions tend to be less accurate. Implantation of fiducials into a patient may be painful and difficult, especially for the C-spine, the implantation process for which may frequently lead to clinical complications. Attempts have therefore been made to develop techniques for fiducial-less tracking.
By using anatomical structures, for example skeletal or vertebral structures, as reference points, the need for fiducial markers (and ensuing surgical implantation) may be eliminated in image-guided surgery. A fiducial-less tracking system and method, which relies on skeletal structures as references, may eliminate the need for fiducial markers.
Typically, in 2D-3D image registration, DRRs (digitally reconstructed radiographs) are reconstructed from the preoperative 3D images, after which image registration is performed between the DRRs and the real-time 2D projection images.
Although fiducial-less tracking that relies on skeletal structures may overcome many of the drawbacks associated with implanted fiducials, these skeletal structures may frequently not be easily visible, or may even be hidden, in the pre-operative DRRs.
Accordingly, there is a need for an improved DRR generation technique, which can increase the contrast, and bring out the details of the reference structures, in the DRRs that are based on the pre-operative scans.